1. Filed of The Invention
The invention relates to an optically readable record carrier of the inscribable type, comprising a preformed track which comprises an information-recording area of a type which is subjected to an optically detectable change when exposed to radiation of an intensity above a specific write intensity.
The invention further relates to an optical read apparatus for reading information recorded in the information-recording area of such a record carrier as a pattern of recording marks with modified optical properties, which read apparatus comprises scanning means for scanning the track by means of a radiation beam, opto-electronic transducer means for converting the radiation beam reflected or transmitted by the track into an electric signal, and a signal-processing circuit for deriving from the electric signal an information signal is representative of the information recorded in the track being scanned.
2. Description of Related Art
Such a record carrier and read apparatus are known from U.S. Pat. No. 4,473,829.
The record carrier described therein comprises an information-recording area formed by a thin metal layer which can be removed by exposure to a high-intensity radiation beam. An information signal can be recorded by modulating the radiation beam in conformity with the information signal while the recording layer is scanned, so that by the local removal of the metal layer a pattern of recording marks of modified reflection is formed between which intermediate areas of unmodified reflection are interposed. When the information thus recorded is read the pattern of recording marks is scanned with a low-intensity radiation beam, the intensity of the reflected radiation beam being modulated in conformity with the pattern being scanned. By means of photodetectors the reflected radiation is converted into an amplitude-modulated electric signal whose modulation depth is proportional to the difference in reflection between the recording marks and the intermediate areas.
In the various types of inscribable record carriers which are currently known the relative difference in reflection between the recording marks and the intermediate areas varies from type to type, which results in variations over a range which is several factors larger than the range which can be handled by the customary read apparatuses. This is mainly caused by the fact that the processing circuits cannot readily handle those large differences in signal strength which result from such large differences in reflection. Therefore the signal-processing circuits must be equipped with comparatively intricate automatic signal-strength controls. Moreover, the response time of such automatic control systems is a drawback, because this results in the first part of the signal to be read not being read reliably.